The present invention relates to apparatus for the clarifying of the liquid portion of a slurry through settling and filtration.
A typical situation in which the liquid portion of a slurry is to be clarified and separated from suspended solids is found in the processing of sugar juices. In the past, such juices were clarified by pouring the juice into a large sedimentation tank, and allowing the juice to remain in the tank a sufficient time for the solid particles to collect in the bottom. Typically, the juice spends about 2 or 3 hours in the tank, and the process is relatively inefficient in that large storage tanks are required to handle the requisite volume. In addition, the fact that the juice sits in the tank with the sediment for a relatively long period of time often causes the juice to take on the coloration of the sediment, decreasing the quality of the juice obtained.
Clarification systems are now under development in which the juice is flocculated prior to clarification. Preflocculation causes the solid particles in the juice to accumulate into larger particles, called flocs. Such flocs are typically heavier than the juice, and fall to the bottom, causing a filtering action on the juice. Such preflocculated juices separate more rapidly into the clarified juice and sediment, and the use of such preflocculated juices in conventional sedimentation tanks is quite inefficient.
Attempts have been made to develop clarifying systems which operate more rapidly than the sedimentation tanks to accommodate preflocculated juices. The patent to Eis, U.S. Pat. No. 3,523,889, discloses such a rapid clarifier. Such clarifiers are produced by Enviro-Clear, a division of Amstar Corporation, and a similar type of system has been tested by the Sugar Research Institute. These rapid clarifiers all employ a large tank which contains a mat of sedimented mud particles with an overlying layer of the clarified juice. The preflocculated juice to be clarified is introduced in the tank at a level below the mud level. The liquid portion of the juice rises through the existing mud while the flocs descend. The rising liquid is thus clarified by settling of the flocs and also the filtering action provided by the mud. Sedimented mud is drawn out of the bottom of the tank, and clarified juice is skimmed off the top.
The rapid clarifiers described above have been plagued by "short circuiting" of the slurry to be clarified. A portion of the slurry injected in the tank rises rapidly through the mud, rather than the slow, gradual ascending movement desired. The rapid rise of such slurry prevents the flocks from settling, and inhibits the filtration action of the sedimented mud. The poorly settled and filtered slurry contaminates the clarified juice collected at the top of the tank. This problem can be reduced by lowering the input volume of the slurry, but only at a substantial reduction in the capacity of the system.